The present invention is related to the field of optical position encoders.
Optical position encoders operate based on patterns of light created by a scale that moves with respect to a light source. Movement of the scale results in corresponding changes in the light pattern, which can be detected and interpreted by optical and electronic components of the encoder to provide an indication of relative position between the scale and the light source.
Optical position encoders typically perform interpolation between adjacent marks or elements of a light pattern (also referred to as “fringes”) in order to obtain higher precision measurements. For example, one class of optical encoders employs diffraction gratings to generate interference patterns having a substantially sinusoidal intensity profile in the direction of relative movement. By sampling the interference pattern at multiple intra-fringe locations and performing standard trigonometric functions on the samples, measurement precision of 14 to 16 bits can be achieved. However, the actual accuracy of such encoders is highly dependent upon the degree to which the light intensity profile is actually sinusoidal. The presence of undesired harmonic components in the intensity profile, for example, can cause significant intra-fringe inaccuracy, reducing the overall accuracy of the encoder.
One example of an optical position encoder employing the above principles employs a light source and a scale in the form of a diffraction grating, and further utilizes a “wavefront compensator” disposed between the light source and the scale. The wavefront compensator generates a plurality of light beams corresponding to different “diffraction orders”. Ideally, a 0th diffraction order is entirely suppressed when the wavefront compensator is of the type known as a “phase grating”. The +/−1st diffraction orders are converged to a location on the scale from which a composite beam is generated that creates the interference pattern on an optical detector. If only the +/−1st diffraction orders are present, it can be shown mathematically that the intensity profile of the interference pattern is theoretically purely sinusoidal. In practice, however, some small amount of the 0th order may also reach the scale, as may some of the higher orders such as the +/−2nd orders etc. All of these unwanted orders introduce distortion into the interference pattern and thus reduce overall encoder accuracy.